Large-scale electrical energy storage is becoming increasingly necessary with the growing implementation of intermittent renewable energy sources such as wind and solar. However, there is a relative lack of viable storage technologies, i.e. hydroelectric water pumping and underground compressed air storage are well established but limited to specific geographic sites, whereas most batteries and ultracapacitors have limited energy storage capacities and times. On the other hand, reversible fuel cells have low round-trip efficiency compared to the above-mentioned technologies.
Energy storage using reversible fuel cells can be understood using FIG. 1, which shows typical current-voltage curves in both fuel cell and electrolysis mode, for two different types of cells. The graph shows current-potential curves in H2/H2O for a proton-exchange membrane (PEM) cell (Pettersson, et al., 2006 J. Power Sources 157, 28-34) and solid oxide cells (than, Z. et al., 2009 Energy Fuels 23, 3089-3096) in both electrolysis (negative current) and fuel cell (positive current) mode. Electrical energy is stored by electrolyzing water, and then electricity is produced using the H2 in fuel cell mode.
Consider first the characteristics of a typical solid oxide electrochemical cell at 800° C. For the gas mixture in this test, the open-circuit potential (OCP) is 0.97 V. Connecting the cell to a load draws a positive current and decreases the voltage—the cell is oxidizing the H2 in the fuel to produce electricity. When the cell is connected to an electrical source that imposes a voltage greater than the OCP, a negative current flows and the device electrolyzes H2O and CO2 to H2 and CO. In the latter process, the electrical energy is stored in chemical form, whereas in the former process chemical energy is converted back to electricity. The above also applies to the other characteristics shown, typical for a reversible PEM cell operating at ˜60° C.
FIG. 2 shows schematically the key components of a renewable fuel production system and an electrical energy storage system, based on the reversible cells discussed above. The arrows and labels show the conversion of electrical energy to a fuel.
In this case, H2O and/or CO2 are supplied as a feedstock for electrolysis in the solid oxide electrochemical cell. In other systems, the resulting fuel is a H2—CO-rich mixture that is subsequently catalytically converted into alcohol or hydrocarbon fuels. FIG. 2 also shows an electricity storage configuration where two tanks are used, one to store a mostly-oxidized gas (predominately H2O and CO2) that is introduced into the solid oxide electrochemical cell for electrolysis (darker arrows), with the resulting reduced gas (predominately H2 and CO) stored in the second tank. This part of the cycle represents storage of electrical energy in chemical form, analogous to charging a battery. The reduced gas is the fuel for fuel cell operation (lighter arrows, analogous to discharging a battery), with the oxidized exhaust stored in the first tank. The oxygen electrode can utilize either ambient air or pure oxygen, the latter is produced during the electrolysis process and can be stored for use during the fuel cell part of the cycle. While the advantage of this system is the substantial improvement in the cell oxygen-electrode performance due to the five times increase in oxygen partial pressure compared to air, the disadvantage is the cost of the oxygen tank and associated hardware. The reversible system depicted in FIG. 2 is very similar to a flow battery.
While either the PEM or solid oxide cells can, in principle, be used in the systems shown in the FIG. 2, there are two limitations with the PEM cells. First, the PEM cells work only with H2/H2O, such that it is more difficult to produce alcohol and hydrocarbon fuels. Second, the low-temperature PEM cells always show a much larger overpotential, as seen by the very rapid change in voltage near zero current in FIG. 1, that is not seen in the solid oxide electrochemical cell. This difference is explained by the high temperature for solid oxide electrochemical cells, which promotes fast electrochemical reactions even without the use of the precious metal electro-catalysts used in the low-temperature cells. The relevance of this difference is seen by calculating the maximum theoretical round-trip efficiency η=VFC/VEL of the reversible cell shown in FIG. 2, where VFC is the cell operating potential in fuel cell mode and VEL is the cell operating potential in electrolysis mode. Assuming that the current density is at least 0.5 A/cm2 in a practical device, the low-temperature cell case provides for η=0.71V/1.81V˜39%, whereas the high-temperature cell provides for η=0.87V/1.07 V=0.81% for ±0.5 A/cm2.
Based on the above efficiency equations, it appears that a solid oxide electrochemical cell can easily yield high η. However, current solid oxide electrochemical cells are also not without their deficiencies; there is a lower limit on the electrolysis voltage VEL due to thermal balance requirements. That is, the electrical energy input into the cell must match the heat requirement of the endothermic electrolysis reactions, which at 800° C. are:H2O═H2(½)O2 ΔH=248.29 kJCO2═CO+(½)O2 ΔH=282.35 kJ
The electrical work done by the cell in electrolyzing a mole of H2O (or CO2) is VELxF, where z=2 for the above reactions and F is Faraday's constant. The electrical energy input matches the thermal requirement for electrolyzing a mole of H2O (or CO2) whenVEL,TN=ΔH/2F, where VEL,TN is the thermal-neutral electrolysis voltage: 1.29 V for H2O and 1.46 V for CO2. If the cell is operated below the thermal-neutral voltage during charging, the reaction will consume heat and the cell will cool uncontrollably. This is illustrated in FIG. 3 for a cell operating at 800° C. on H2 with a steam content near 50% first in electrolysis mode and then in fuel cell mode. At this condition, VTN=1.29 V, but to achieve 80% electrical efficiency, VEL=1.085 V and VFC=0.868 V. Because the electrolysis voltage is below the thermal-neutral voltage, the cell consumes heat during the charge cycle and will cool. Similarly, as the system discharges, the thermal-neutral voltage is considerably higher than the operating voltage and excess heat is generated. Under these conditions, the temperature of the cell cannot be maintained without an external heat source in electrolysis mode and substantial active cooling in fuel cell mode. A substantial amount of heat energy is wasted, unless heat storage can be implemented to use the heat during the next electrolysis cycle.
Thus, to maintain the cell temperature during electrolysis mode, the cell must be operated above the thermal-neutral voltage during charging and the efficiency is reduced to η=0.88V/1.29 V=68% for H2O, and an even lower value for CO2. Similar conclusions can be drawn for the fuel-production case shown in FIG. 2.
It is therefore desirable to provide a method for improving the efficiency and durability of electrical energy storage to cure the deficiencies presented above.